Reduction of carbonyl compounds by a silane in the presence of a zinc catalyst

ABSTRACT

A reductive system including a silane, preferably PMHS, and an active zinc compound, which is monomeric and not a hydride, wherein a reduction of a carbonyl substrate to a corresponding alcohol is achievable.

BRIEF SUMMARY OF THE INVENTION

[0001] The present invention concerns the field of organic synthesis. Itconcerns, more particularly, a process for the selective reduction ofcarbonyl compounds, such as aldehydes, ketones, esters and lactones intothe corresponding alcohols, using silanes as reducing agents, preferablypolymethylhydrosiloxane (PMHS), in the presence of catalysts whichcomprise monomeric zinc compounds, complexed by basic ligands such asamines, polyamines, aminoalcohols, amine oxydes, amides, phosphoramides,etc..

BACKGROUND OF THE INVENTION

[0002] The selective reduction of carbonyl compounds to thecorresponding alcohols in the course of which only the reaction of theC═O function is observed, is an important task in the field of organicchemistry. Until now, there were exclusively used hydride reducingagents, such as lithium aluminum hydride LiAlH₄, sodium borohydrideNaBH₄, or sodium dihydroxybis(2-methoxyethoxy)aluminate (SDMA) offormula NaAlH₂(OCH₂CH₂OCH₃)₂, the two latter reagents being of limitedvalue for the reduction of esters and lactones. All the above-mentionedreagents are employed in stoichiometric amounts and show thedisadvantage of releasing hydrogen in the course of the reaction or,when entering into contact with humidity, of leading to explosion risksand requiring the inertization of the reactors used. Furthermore, theuse of these reagents is costly as they are required in stoichiometricamounts. Thus, there is a continous search for other systems which aremore economic and easier to use.

[0003] Several publications describe the use of silanes as reducingagents for carbonyl substrates, together with a metal catalyst. Apreferred silane for this type of reductions is polymethylhydrosiloxaneor PMHS, according to the general formula

[0004] U.S. Pat. No. 3,061,424 to Nitzsche and Wick describes thereduction of aldehydes and ketones with PMHS and a salt of mercury,iron, copper, titanium, nickel, zirconium, aluminum, zinc, lead, cadmiumand, as the preferred embodiment, tin. This reductive system requiresactivation by a proton source, without which the reaction does notproceed. However, the system is not effective for the reduction ofesters and lactones.

[0005] U.S. Pat. No. 5,220,020 to Buchwald et al. describes a method forthe preparation of alcohols by the reduction of carbonyl compounds usinga system composed of a silane reducing agent and a metal catalyst offormula M(L)(L′)(L″) to M(L)(L′)(L″)(L′″)(L^(IV))(L^(V)), in which M isa metal belonging to any of groups 3, 4, 5, or 6 of the periodicaltable, a lanthanide or an actinide, whereas (L′) to (L^(V)) representhydrogen, an alkyl group, an aryl group, a silyl group, a halogen atom,or a —OR, —SR or —NR(R′) group, R and R′ being hydrogen, an alkyl or anaryl group. Amongst the preferred catalysts, the cited patent mentionstitane (IV) isopropylate or ethylate or trichlorotitane (IV)isopropylate. Such a system is said to be appropriate for the reductionof esters, lactones, amides or imines. More recently, Breedon andLawrence (Synlett., 1994, 833) and Reding and Buchwald (J. Org. Chem.,1995, 60, 7884) have described a similar process, namely the use ofnon-activated titane tetraalkoxydes as catalysts for the reduction ofesters by PMHS. The method described in those three mentioned referencesrequires the use of large amounts, at least 25 mole % with respect tothe substrate, of catalyst. Barr, Berk and Buchwald (J. Org. Chem.,1994, 59, 4323) have shown that the complex Cp₂TiCl₂, when reduced bybutyllithium or ethylmagnesium bromide, could catalyze the reduction ofesters into the corresponding alcohols with good yields, but thistechnique requires reagents which are expensive and difficult to use ina large scale, as is the case in industrial organic synthesis.

[0006] As closest prior art, there should be cited the internationalapplication WO 96/12694 of the applicant, describing the reduction ofaldehydes, ketones, esters and lactones by a reductive system composedof silanes and a metal hydride, leading to the corresponding alcoholswith good yields. This systems requires only very low amounts ofcatalyst, i.e. the metal hydride, in the order of 1 mol % with respectto the substrate. The hydride is formed by the reaction of a salt of therespective metal with an appropriate reducing agent, preferably NaBH₄.Besides zinc salts, cobalt manganese and iron salts are used asprecursors for the generation of metal hydrides. According to anotherpreferred embodiment, PMHS is used as silane reducing agent.

DETAILED DESCRIPTION OF THE INVENTION

[0007] We have now successfully developped a process for the reductionof carbonyl compounds with silanes, catalyzed by metal derivatives whichare not hydrides and which, in consequence, do not require the use of areducing agent like, for example, NaBH₄.

[0008] The object of the invention is a process for the preparation ofalcohols by reduction of the carbonyl function in substrates belongingto the class of aldehydes, ketones, esters or lactones, which substratesmay contain unsaturated functions other than the carbonyl group, wherein

[0009] a) said carbonyl substrate is reacted with an effective amount ofa silane, preferably PMHS, in the presence of catalytic amounts of anactive zinc compound which is monomeric and not a hydride, to form asiloxane,

[0010] b) the thus-obtained siloxane is hydrolyzed with a basic agent toform an alcohol, and

[0011] c) the resulting alcohol is separated and purified, if necessary.

[0012] Another object of the invention is a reductive system comprising

[0013] a) a silane, preferably PMHS, and

[0014] b) an active zinc compound which is monomeric and not a hydride.

[0015] The present invention is based on the surprising fact that theuse of a monomeric species of zinc considerably enhances the reactivityof a reductive system for carbonyl compounds comprising a silane and azinc compound. Thus, reductive systems comprising a zinc salt and asilane, as described in U.S. Pat. No. 3,061,424 to Nitzsche and Wickwhich has been cited beforehand, are by far less reactive than thesystem according to the present application. In particular, the systemas described in the prior art is not capable of reducing esters andlactones, in contrast to the reductive system of the present invention.

[0016] On the other hand, although the above-cited document WO 96/12694of the applicant shows that it is possible to enhance the reactivity ofa silane for the reduction of carbonyl substrates by adding zinc saltsor complexes, the latter require the activation by a reducing agent. Asreducing agent, compounds like NaBH₄, LiAlH₄, lithium or aluminum alkylsor Grignard compounds were used to generate a highly reactive species,namely a hydride.

[0017] The present invention, however, uses zinc compounds such as saltsor complexes which do not require the activation by a reducing agent andwhich, when employed in stoichiometric amounts and together with asilane, catalyze the reduction of all sorts of carbonyl compounds.

[0018] The chemistry of zinc is in general characterized by the tendencyof the metal to reach a coordination number higher than 2 which is aconsequence of its valence state +2. The zinc can reach the highercoordination number it desires to attain by oligo- or polymerization,after which in general a tetra- or hexacoordination is observed. Forthose reasons, zinc salts or complexes are in most cases oligo- orpolymeric, and as examples, there are mentioned here zinc carboxylatesand halides.

[0019] However, an electronically unsaturated class of compounds aredialkyl- and diaryl zinc compounds. They are not capable of reaching ahigher coordination number than 2 by oligo- or polymerization becausealkyl and aryl groups cannot act as bridging ligands. Dialkyl- anddiaryl zinc compounds are therefore monomeric, and they show a linearstructure.

[0020] We have established that all the above-mentioned compounds showeither no activity or a very low activity when used for the reduction ofcarbonyl compounds. However, these poly- or oligomeric species as wellas dialkyl- or diaryl zinc compounds, when treated with an appropriatecomplexing agent which is capable of generating a monomeric activespecies, become highly effective catalysts for the reduction ofaldehydes, ketones, esters and lactones by a silane.

[0021] According to the invention, there can be used an oligo- orpolymeric precursor compound or a dialkyl- or diaryl zinc compound,which is converted into an active salt or complex by treatment with anappropriate complexing agent. Moreover, we have found that there canalso be used known monomeric complexes or salts which turned out to beactive in the process of the invention, but whose activity has passedcompletely unnoticed until now.

[0022] As the precursor compound, practically any known compound of zincaccording to the general formula ZnX₂ can be used. In this formula, Xstands for any anion. Preferred anions X are defined below.

[0023] The active catalyst of the invention can be described by thegeneral formula ZnX₂L_(n). The catalyst can be obtained in situ, in thereaction medium, or be prepared separately from a zinc compound such as,for example, a salt or complex of general formula ZnX₂, mentioned above.In the formula ZnX₂ of the precursor compound and ZnX₂L_(n) of theactive catalyst, X is preferably any anion selected from the groupconsisting of carboxylates, β-diketonates, enolates, amides,silylamides, halides, carbonates and cyanides and organic groups such asalkyl, cycloalkyl, alkoxy, aryl, aryloxy, alkoxyalkyl, alkoxyaryl,aralkoxy, aralcoyl and alkylaryl groups. Amongst this group, one willpreferably use a zinc carboxylate of formula Zn(RCO₂)₂ like, forexample, the acetate, propionate, butyrate, isobutyrate, isovalerate,diethylacetate, benzoate, 2-ethylhexanoate, stearate or naphthenate; azinc alkoxyde of formula Zn(OR)₂, wherein R is an alkyl group from C₁ toC₂₀, preferably from C₁ to C₅ such as, for example, the methoxyde,ethoxyde, isopropoxyde, tert-butoxyde, tert-pentoxyde, or the8-hydroxyquinolinate; a zinc β-diketonate like, for example, theacetylacetonate, substituted or unsubstituted, or the tropolonate acompound of the type alkylzinc, arylzinc, alkyl(alkoxy)zinc oraryl(alkoxy)zinc comprising from 1 to 20 carbon atoms, preferably from 1to 5 carbon atoms or a derivative thereof such as, for example,dimethylzinc, diethylzinc, dipropylzinc, dibutylzinc, diphenylzinc,methyl(methoxy)zinc or methyl(phenoxy)zinc, or a derivative of the typehalide(alkyl)zinc.

[0024] In the formula ZnX₂L_(n), n is an integer from 1 to 6. Theligands L can be identical or different and be selected from the groupconsisting of amines, polyamines, imines, polyimines, aminoalcools,amines oxydes, phosphoramides and amides.

[0025] The amine may be a primary, secondary, or tertiary aliphatic,alicyclic or aromatic amine comprising from 2 to 30 carbon atoms.Non-limiting examples include aniline, triethylamine, tributylamine,N,N-dimethylaniline, morpholine, piperidine, pyridine, picolines,lutidines, 4-tertiobutylpyridine, dimethylaminopyridine, quinoline andN-methylmorpholine.

[0026] The polyamines may comprise from 2 to 6 primary, secondary ortertiary amine groups, and from 2 to 30 carbon atoms such as, forexample, ethylenediamine, 1,2- and 1,3-propylenediamine 1,2-, 1,3- and1,4-butanediamine, hexamethylenediamine, N,N-dimethylethylenediamine,diethylenetriamine, dipropylenetriamine, triethylenetetramine,tetramethylethylenediamine, N,N-dimethylpropylenediamine,N,N,N′-trimethylethylenediamine,N,N,N′,N′-tetramethyl-1,3-propanediamine, hexamethylenetetramine,diazabicyclononane, sparteine, orthophenantroline, 2,2′-bipyridine andneocuproine.

[0027] The aminoalcohols may comprise one or several primary, secondaryor tertiary amine functions together with one or several primary,secondary or tertiary alcohol functions like in, for example,ethanolamine, diethanolamine, triethanolamine, dimethylaminoethanol,diethylaminoethanol, dimethylaminomethanol, diethylaminomethanol,2-aminobutanol, ephedrine, prolinol, valinol, cinchonidine, quinine andquinidine.

[0028] As ligands belonging to the family of imines or diimines andcapable of activating zinc derivatives or compounds in the context ofthe present invention, one can cite, as non-limiting examples, thecompound families according to formulae [I] to [V] below, in which thegroups R₁ to R₆ each represent a hydrogen atom or an alkyl, cycloalkyl,alkoxy, aryl, aryloxy, alkoxyalkyl, alkoxyaryl, aralkoxy, aralcoyl,alkylaryl or aralkyl goup comprising from 1 to 20 carbon atoms.

[0029] Other ligands capable of activating zinc compounds andderivatives yet include amides like, for example, dimethylformamide,dimethylacetamide or N-methyl-pyrrolidone, phosphoramides such as, forexample, hexamethylphosphortriamide, phosphine oxides like, for example,triphenylphosphine oxide, tributyl- or trioctylphosphine oxide, amineoxides like, for example, pyridine N-oxyde, 4-picoline-N-oxyde,N-methyl- morpholine N-oxyde and sulfoxydes like, for example, dimethyl-or diphenylsulfoxyde.

[0030] The invention also concerns monomeric zinc complexes which turnedout to be active in the process of the invention. A preferred class ofcompounds are monomeric zinc carboxylates. This class of molecules isnot described in the chemical literature, with the exception of thecompound Zn(O₂CCH₃)₂(pyridine)₂, see J. Am. Chem. Soc. 119, 7030,(1997).

[0031] As preferred compounds amongst these complexes, there are citedhere [Zn(benzoate)₂(Me₂NCH₂CH₂OH)₂],[Zn(diethylacetate)₂(2,2′-bipyridyl)], [Zn(diethyl-acetate)₂(1,2-diaminopropane)₂] and [Zn(benzoate)₂(TMEDA)](TMEDA=tetramethyl-ethylenediamine). The preparation andcharacterization of these compounds is described below.

[0032] A great number of silanes can be used in the process according tothe present invention. Such silanes are known to a person skilled in theart, and they will be chosen according to their capacity to effectivelyreduce carbonyl substrates in the process according to the presentinvention. As non-limiting examples, there can be cited trialkylsilanes,dialkylsilanes or trialkoxysilanes. More specific examples includedimethylsilane, diethylsilane, trimethoxysilane and triethoxysilane.There will preferably be used PMHS due to its effectiveness,availability and price.

[0033] The procees according to the present invention is lined out inthe following reaction schemes, which apply to the particular andpreferred case of employing PMHS as reducing agent.

[0034] The concentration of the catalyst ZnX₂L_(n), given in mole % withrespect to the substrate, is generally from 0.1 to 10%, preferably from1 to 5%.

[0035] There will typically be consumed 2 mole equivalents of PMHS perester or lactone function, and one equivalent for the reduction ofaldehydes and ketones. For practical reasons, there will preferably beused a slight excess of PMHS with respect to these stoichiometricamounts, in general of the order of 10 to 40% excess, based on thestoichiometric quantity. The reduction reaction according to theinvention also takes place when the silane is used in sub-stoichiometricamounts, but this results in a decrease in conversion. According to theinvention, therefore, the term “effective amount” means an amount ofsilane sufficient to induce reduction of the substrate.

[0036] The alcohol which is obtained as reaction product can be obtainedby hydrolysis of the formed polysilylether. This hydrolysis may becarried out by adding to the reaction mixture an aqueous or alcoholicsolution of a basic agent such as, for example, sodium or potassiumhydroxide, lime or sodium or potassium carbonate. The ratio of the basewith respect to the PMHS used will be from about 1 to 2 moleequivalents. After complete hydrolysis, there will in general beobserved the formation of two phases. The desired alcohol is found inthe organic phase and can be obtained by evaporation of the solventwhich may be present. The obtained residue may be distilled for furtherpurification.

[0037] The reduction can be carried without a solvent or in a solventsuch as, for example, an ether (e.g. methyltert-butylether,diisopropylether, dibutylether, tert-amyl-methylether, tetrahydrofuraneor dioxane), an aliphatic hydrocarbon (e.g. heptane, petroleum ether,octane, or cyclohexane) or an aromatic hydrocarbon (e.g. benzene,toluene, xylene or mesitylene), or mixture thereof.

[0038] As laid out above, the reduction according to the invention isapplicable to various carbonyl compounds which may contain unsaturatedfunctionalities other than the carbonyl group such as, for example,olefin, acetylene, nitrile or nitro groups which will not be affected bythe reduction reaction.

[0039] As non-limiting examples for aldehyde substrates, there can becited butanal, pentanal, heptanal, octanal, decanal, dodecanal, linearor branched. Other aldehydes which are unsaturated and which can beselectively reduced into the corresponding unsaturated alcohols includeacroleine, methacroleine, prenal, citral, retinal, campholene aldehyde,cinnamic aldehyde, hexylcinnamic aldehyde, formylpinane and nopal.Aromatic aldehydes like, for example, benzaldehyde, cuminic aldehyde,vanilline, salicylaldehyde or heliotropine are also easily reduced tothe corresponding alcohols.

[0040] As non-limiting examples for saturated and unsaturated ketoneswhich can be reduced into the corresponding alcohols by silanesaccording to the invention, there can be cited hexan-2-one, octan-2-one,nonan-4-one, dodecan-2-one, methylvinylketone, mesityl oxide,acetophenone, cyclopentanone, cyclododecanone, cyclohexen-1-en-3-one,isophorone, oxophorone, carvone, camphor, beta-ionone, geranylacetoneand 2-pentylcyclopenten-2-one.

[0041] As non-limiting examples for saturated and unsaturated esters orlactones which can be reduced into the corresponding alcohols by silanesaccording to the invention, there can be cited acetates, propionates,butyrates, isobutyrates, benzoates, acrylates and crotonates,cinnamates, cis-3-hexenoates, sorbates, salicylates, 10-undecylenates,oleates, linoleates, any ester of natural fatty acids and mixturesthereof. All the above-cited esters may, for example, be alkyl or arylesters, preferably methyl esters. Other non-limitative examples includelactones, such as ε-caprolactone, decalactone, dodecalactone, diketeneand sclareolide.

[0042] A remarkable property of the catalysts according to the inventionis that they allow the reduction of natural triglycerides of fattyacids, like those which form the vegetable and animal oils. In thecourse of the reaction of a mixed triglyceride derived from distinctfatty acids, there can be obtained simultaneously saturated andunsaturated natural alcohols without any modification of the position orof the stereochemistry of the olefinic double bonds. This is ofparticular value for olefinic bonds showing a cis-configuration.

[0043] In the above Scheme (3), the substituents R₁, R₂and R₃ arehydrocarbon groups which can be identical or different and which cancontain from 1 to 20 carbon atoms. In the case where these groupscontain one or more olefinc groups of a defined stereochemistry (which,in general, will be cis), the corresponding alcohol obtained afterreduction according to the invention will have the same stereochemistry.Thus, oils rich in linoleic and/or linolenic acid, like linseed oil,will be transformed into mixtures rich in linoleyl and/or linolenylalcohol. Conventional hydrogenation of these vegetable oils is generallycarried out at high pressures and temperatures, in contrast with thepresent invention. Furthermore, because there are used in theseconventional hydrogenations the methyl esters of the respective acidsobtained by transesterification of the oils with methanol, there is inmost cases observed a modification of the stereo- chemistry of theprecursor fatty esters in the course of the transesterification and thehydrogenation reaction.

[0044] Amongst the triglycerides which can be reduced by the processaccording to the invention, there can be cited, as non-limitingexamples, trioleine, peanut oil, soya oil, olive oil, colza oil, sesameoil, grape-seed oil, linseed oil, cacao butter, palm oil, palm-kerneloil, cotton oil, copra oil, coconut oil, and pork, beef, mutton andchicken fat.

[0045] Other oils and fats which are found in nature and which are nottriglycerides, but esters of unsaturated fatty acids and monovalentunsaturated alcohols, like jojoba oil and sperm oil, can also be reducedaccording to the present invention, without any modification of theposition or of the stereochemistry of the double bonds present in theester molecule.

[0046] The reaction temperature can vary within a wide range of values,and will in general be in the range of from −50° C. to 250° C. Thetemperature chosen will depend on the reactivity of the substrate andcan be adjusted accordingly without difficulty. More generally, thereaction will be carried out at a temperature within the range of from50 to 110° C.

[0047] The invention will now be illustrated in greater detail in thefollowing examples in which the temperatures are indicated in degreescentigrade, the yields in mole %, the chemical shift δ of the NMR datain ppm, relative to tetramethylsilane as internal reference, and theabbreviations have the usual meaning in the art.

[0048] Embodiments of the Invention

EXAMPLE 1 Synthesis of the complex [Zn(benzoate)₂(Me₂NCH₂CH₂OH)₂]

[0049] The compound was prepared as described below and illustrated inscheme (4)

[0050] To a suspension of 3.06 g (10 mmol) of zinc benzoate in 50 ml ofdichloromethane there were added 1.8 g (20 mmol) ofdimethylaminoethanol. An exothermic reaction, followed by completesolution of the zinc benzoate, was observed. After 1 h of stirring at20° C., the solvent was evaporated, and the solid residue obtained wascrystallized from a minimum amount of dichloromethane. There wereobtained 3.9 g (80%) of the desired complex as white solid crystals, thestructure of which could be obtained by X-ray structure analysis from asingle crystal.

[0051] NMR(¹H): δ_(H): 2.4(12H, s); 2.65(2H, t, CH₂—N); 3.85(2H, t,CH₂—O); 7.35-7.5(m, 6H, arom.); 8.1-8.2(d, 4H, arom.);

[0052] NMR(¹³C): 46.37(q, CH₃); 57.34(t, CH₂—N); 61.02(t, CH₂—O);127.88(d); 129.9(d); 131.19(d); 135.36(s); 174.36(s, CO₂—).

EXAMPLE 2 Synthesis of the complex [Zn(diethylacetate)₂(2,2′-bipyridyl)]

[0053] This compound was prepared as described below, according toscheme (5)

[0054] 3 g (10 mmole) of zinc diethylacetate were dissolved in 50 ml ofdiisopropylether. There were then added 10 mmole of the ligand2,2′-bipyridyl, and the mixture was then stirred at 20° C. A precipitaterapidly formed, which was isolated by filtration and recristallized fromcyclohexane. The yield was 80%.

[0055] M.p.: 135° C. Analysis: C₂₂H₃₀N₂O₄Zn; calculated: C, 58.48; H,6.69; N, 6.20; found: C, 58.6; H, 6.6; N, 6.15

[0056] NMR(¹H): δ_(H): 0.85(12H, t, CH₃); 1.45(4H, m, CH₂—); 1.60(4H, m,CH₂—), 2.21(2H, m, CH═), 7.6(m, 2H, arom.), 8.05(m, 2H, arom.), 8.21(m,2H, arom.), 9.03(m, 2H, arom.)

[0057] NMR(¹³C): 12.13(q, CH₃); 25.75(t, CH₂—), 50.01(d, CH═);121.02-149.91(d,d,d,d,s, arom.); 185.47(s, CO₂—).

EXAMPLE 3 Synthesis of the complex[Zn(benzoate)₂(tetramethylethylenediamine)]

[0058] This compound was prepared as described below and outlined inscheme (6)

[0059] The reaction was carried out as described in example 1, using 1equivalent of tetramethylethylenediamine instead of the 2 equivalents ofdimethylaminoethanol. Yield: 85%.

[0060] NMR(¹H): δ_(H): 2.62(12H, s, CH₃N); 2.77(4H, s, CH₂—N);7.3-7.5(m, 6H, arom.); 8.1(d, 4H, arom.);

[0061] NMR(¹³C): 46.57(q, CH₃N); 56.60(t, CH₂—N); 127-131(d,d,d);133.8(s); 175(s, CO₂—).

EXAMPLE 4 Synthesis of the complexe[Zn(diethylacetate)₂(1,2-diaminopropane)₂]

[0062] This compound was prepared as described below, according to thescheme (7)

[0063] The reaction was carried out as described in example 2, using 2equivalents of 1,2-diaminopropane instead of 1 equivalent of2,2′-bipyridyl. Yield=75%.

[0064] M.p.: 148° C. Analysis: C₁₈H₄₂N₄O₄Zn; calculated: C, 48.70; H,9.54; N, 12.62; found: C, 48.6; H, 9.6; N, 12.5

[0065] NMR(¹H): δ_(H): 0.88(12H, t, CH₃); 1.13(6H, d, CH₃); 1.48(8H, m,CH₂—), 2.0(2H, m, CH═), 2.4(m, 2H), 2.8-3.5(m, 12H, NH₂), 8.21(m, 2H,arom.), 9.03(m, 2H, arom.)

[0066] NMR(¹³C): 12.57(q, CH₃); 21.44(q, CH₃), 26.05(t, CH₂);45.73(t,CH₂); 46.61(d, CH═); 52.27(d, CH═); 77.29(d, CH═); 183.30(s,CO₂—).

[0067] Reduction Reactions

EXAMPLE 5

[0068] Into a three-necked 250 ml flask were charged 30 g of isopropylether and 27.2 g of methyl benzoate (0.2 mole), followed by 4 mmole ofthe crystalline complex prepared according to example 2, e.g.[Zn(diethylacetate)₂(2,2′-bipyridyl)]. The mixture was heated to 70° C.(reflux) before adding 30 g of PMHS (0.44 mole) over 15 minutes. Themixture was stirred for one further hour under reflux until completedisappearance of the substrate (monitored by GC analysis). The mixturewas then cooled to 20° C. before adding 66 g of an aqueous 45%KOH-solution (0.52 mole) with rapid stirring, followed by furtherstirring for 1 h. There were then added 100 g of water, and the mixturewas decanted. The aqueous phase containing the potassiumpolymethylsiliconate was decanted, then the organic phase was washedwith 50 ml of water. The solvent was removed by distillation, to obtain21 g of crude product. The distillation from residues gave 20.5 g ofbenzyl alcohol in a purity greater than 98% (yield=95% ).

EXAMPLE 6 (COMPARATIVE)

[0069] The reaction was carried out as in example 5, with the exceptionthat 1.12 g (4 mmol) of polymeric zinc diethylacetate were used ascatalyst. After 4 h, no reaction of the employed methyl benzoate couldbe observed, indicating that the presence of an appropriate ligand isessential for the depolymerisation reaction and hence the activation ofthe zinc diethylacetate for the reduction of the ester.

EXAMPLE 7 TO 23

[0070] These examples, summarized in table 1, illustrate theconsiderable influence that the addition of bidentate ligands has on thecatalytic activity of zinc carboxylates in the reduction of methylbenzoate to benzyl alcohol by PMHS. The reaction conditions, resemblingthose of example 5, are given at the end of the table. This table alsogives the position of the infrared bands ν(CO₂)_(as) and ν(CO₂)_(s) ofthe carboxylate groups of the isolated complexes, which makes itpossible to identify the depolymerization of the precursor zinccarboxylate before it attains its catalytic activity. TABLE 1 Reductionof methyl benzoate to benzyl alcohol. Influence of the nature of thebidentate ligand. Infrared Yield ν(CO₂)_(as) benzyl Zn CarboxylateLigand ν(CO₂)_(s) alcohol Example 2 mole % 2 mole % cm⁻¹ mole % 7[Zn(benzoate)₂]_(n) — 1639, 1530 0 1417 8 [Zn(2-Et hexanoate)₂]n — 1631,1554 0 1417 9

— 1539 1397 90 10

— 1553 1398 85 11 [Zn(diethylacetate)₂]_(n)

1595 1421 97 12 [Zn(diethylacetate)₂]_(n)

1549 1413 97 13 [Zn(diethylacetate)₂]_(n)

— 84 14 [Zn(diethylacetate)₂]_(n)

1555 1407 93 15 [Zn(diethylacetate)₂]_(n)

1605 1400 95 16 [Zn(diethylacetate)₂]_(n)

1603 1384 96 17 [Zn(diethylacetate)₂]_(n)

1564 1422 97 18 [Zn(2-Et hexanoate)₂]_(n)

— 98 19 [Zn(diethylacetate)₂]_(n)

1600 1401 98 20 [Zn(diethylacetate)₂]_(n)

1606 1420 97 21 [Zn(diethylacetate)₂]_(n)

— 96 22 [Zn(diethylacetate)₂]_(n)

1599 1425 97 23 [Zn(diethylacetate)₂]_(n)

— 94

[0071] Reaction conditions: Methyl benzoate=20 mmole, PMHS=44 mmole,An(carboxylate)₂=0.4 mmole, Ligand=0.4 mmole (if not indicatedotherwise). Solvent=diisopropylether (2 ml), 70° C., 4 h, Et=ethyl.

EXAMPLES 24 TO 30

[0072] These examples, summarized in table 2, illustrate theconsiderable influence of the addition of monodentate ligands on thecatalytic activity of zinc carboxylates in the reduction of methylbenzoate by PMHS. The reactions were carried out as describedbeforehand, using methyl benzoate as substrate and 2 mole % of zincdiethylacetate together with 4 mole % of the monodentate ligand. TABLE 2Reduction of methyl benzoate by PMHS in the presence of zinccarboxylates complexed by monodentate ligands Ex- Yield am- ZnCarboxylate Ligand PhCH₂OH ple 2 mole % 4 mole % mole % 24[Zn(diethylacetate)₂]_(n) Triethylamine 55 25 [Zn(diethylacetate)₂]_(n)Morpholine 28 26 [Zn(diethylacetate)₂]_(n) Piperidine 48 27[Zn(diethylacetate)₂]_(n) 4-tertiobutylpyridine 88 28[Zn(diethylacetate)₂]_(n) Hexamethylphosphortriamide 96 29[Zn(diethylacetate)₂]_(n) Trioctylphosphine oxyde 38 30[Zn(diethylacetate)₂]_(n) Dimethylsulfoxyde 98

EXAMPLES 31 TO 36

[0073] These examples show that the favorable influence of the additionof the ligands specified beforehand is also existant with respect to thecatalytical activity of zinc β-diketonates, like acetylacetonate, forthe reduction of esters using PMHS. It is known that zincacetylacetonate has a trimeric structure which becomes monomeric andoctahedric when it is reacted with bidentate ligands, like2,2′-bipyridine.

[0074] Table 3 below shows that zinc acetylacetonate on its ownpossesses a low activity in the reduction of esters by PMHS.

[0075] The addition of 1 equivalent of a primary or secondary diamine tozinc acetylacetonate allows to obtain zinc complexes capable ofcatalyzing the complete conversion of methyl benzoate to thecorresponding alcohol. TABLE 3 Reduction of methyl benzoate by PMHS inthe presence of zinc acetylacetonate complexed by various ligands Zincprecursor Yield compound Ligand PhCH₂OH Example 2 mole % 2 mole % mole %31 [Zn(acac)₂]₃ — 20 32 [Zn(acac)₂]₃

98 33 [Zn(acac)₂]₃

97 34 [Zn(acac)₂]₃

95 35 [Zn(acac)₂]₃

75 36 [Zn(acac)₂]₃

53

EXAMPLES 37 TO 42

[0076] In these examples, there will be shown that the favorableinfluence of the addition of the ligands specified beforehand is alsoexistant with respect to the catalytical activity of dialkylzinccompounds, like diethylzinc, for the reduction of esters using PMHS(Table 4). Dialkylzinc compounds have a monomeric linear structure witha C—Zn—C angle which is 180° and are unreactive under the conditions ofthe invention. In the presence of a bedentate ligand L, like a tertiarydiamine, they form a monomeric complex of tetrahedral structure ZnR₂L[see O'Brien et al., J. Organomet. Chem., 1993, 449, 1 et 1993, 461, 5].TABLE 4 Reduction of methyl benzoate by PMHS in the presence ofdiethylzinc complexed by various ligands Yield Zinc precursor compoundLigand PhCH₂OH Example 2 mole % 2 mole % mole % 37 ZnEt₂ —  0 38 ZnEt₂

75 39 ZnEt₂

98 40 ZnEt₂

94 41 ZnEt₂

97 42 ZnEt₂ 4-tert-butyl pyridine 95 (4 mole %)

EXAMPLES 43 TO 47

[0077] In these examples, there will be shown that the favorableinfluence of the addition of the ligands specified beforehand is alsoexistant with respect to the catalytical activity of zinc alcoxydes forthe reduction of esters using PMHS. Table 5 shows that the zinctert-pentoxylate, formed in situ by the addition of 2 equivalents ofpotassium tert-pentoxyde (in toluene solution) to one equivalent ofanhydrous zinc chloride does not show a pronounced activity for thereduction of methyl benzoate by PMHS, whereas the addition of primary,secondary and tertiary diamines results in highly active catalysts.TABLE 5 Reduction of methyl benzoate by PMHS in the presence of zincalcoxydes complexed by various ligands Zinc precursor Yield compoundLigand PhCH₂OH Example 2 mole % 2 mole % mole % 43 Zn(OC₅H₁₁)₂ — 51 44Zn(OC₅H₁₁)₂

99 45 Zn(OC₅H₁₁)₂

99 46 Zn(OC₅H₁₁)₂

97 47 Zn(OC₅H₁₁)₂

95

EXAMPLES 48 TO 52

[0078] Reactions were carried out as described in example 5, inrefluxing diisopropyl ether, and using a mixture containing 2 mole % ofzinc diethylacetate and 2 mole % of dimethylaminoethanol, each withrespect to the substrate. There were used 20 mmoles of the respectiveester which was reduced with 44 mmoles of PMHS. Hydrolysis was carriedout when the substrate had disappeared, using 60 mmoles of KOH (in theform of an aqueous 45% KOH solution). After decantation and evaporationof the solvent, the formed alcohol was distilled. In all cases, thestereochemistry of the starting compound was not affected, as shown bythe results presented in Table 6. TABLE 6 Reduction of different estersby PMHS in the presence of zinc diethylacetate complexed bydimethylaminoethanol Yield Example Substrate Product mole % 48

95 49

91 50

94 51

97 52

94

EXAMPLES 53 TO 59

[0079] Reactions were carried out as described in example 5, inrefluxing diisopropyl ether, and using a mixture containing 2 mole % ofzinc diethylacetate and 2 mole % of one of the ligands mentioned inTable 7 below, each with respect to the substrate. As substrates, therewere used 20 mmoles of the respective aldehyde or ketone, which wasreduced with 22 mmoles of PMHS. Hydrolysis was carried out after thesubstrate had completely disappeared, using 60 mmoles of KOH (in theform of an aqueous 45% KOH solution). After decantation and evaporationof the solvent, the alcohol formed was distilled. The results in Table 7show that, in all cases, the reduction of aldehydes and ketonesproceeded with excellent yields, without any modification of thestereochemistry of the starting compound. TABLE 7 Reduction of differentaldehydes and ketones by PMHS in the presence of zinc diethylacetatecomplexed by various ligands Example Ligand Substrate Product Yield 53

95 54

88 55

93 56

95 57

94 58

90 59

95

EXAMPLES 60 TO 62

[0080] The reactions were carried out as indicated in example 5 andusing ZnF₂ as catalyst. The results show that zinc halides are active inthis type of reduction. TABLE 8 Reduction of methyl benzoate by PMHS inthe presence of ZnF₂ complexed by various ligands Yield Zinc precursorcompound Ligand PhCH₂OH Example 2 mole % 2 mole % mole % 60 ZnF₂ —  0 61ZnF₂

97 62 ZnF₂

93

EXAMPLE 63

[0081] Reduction of peanut oil

[0082] A three-necked 1 l flask was charged with 200 ml of toluene, 11 gof zinc 2-ethylhexanoate (0.03 mol) and 5.34 g (0.06 mol) ofdimethylaminoethanol. There were then added 200 g of peanut oil and themixture was heated to reflux (110° C). 200 g (0.5 mol) of PMHS wereadded over 1 h, and the mixture was kept under reflux for another 2 h.After this time, GC analysis carried out on samples hydrolyzed by a 30%methanolic KOH solution showed that the amount of alcohol in thereaction mixture was constant. The mixture was then poured into 450 g ofa 30% methanolic KOH solution and then kept for 1 further hour at 50° C.There were then added 300 g of water and the mixture decanted. Thesolvent was then evaporated from the organic phase and the residuedistilled at 200-250° C./1 hPa to obtain 100 g of a mixture containing14% of 1-hexadecanol, 55% of oleyl alcohol and 17% of linoleyl alcohol.

EXAMPLE 64

[0083] Reduction of ethyl sorbate

[0084] A 1 l three-necked flask equipped with a reflux condenser, innerthermometer, syringe pump and magnetical stirrer, was charged with 13.3g (4 mole % relative to the substrate) of Zn(2-ethylhexanoate)₂, 4 g (4mole %) of dimethylaminoethanol, 10 ml of toluene and heated to 80°.There were then added 210.1 g (1.5 mole) of ethyl sorbate, 0.42 g of BHT(2,4-di-tert-butyl-p-cresol), toluene (ca 200 ml) and the solution wasbrought to reflux. 213 g (corresponding to 2.1 equivalents) of PMHS werethen added over 90 min, and the reaction mixture was then heated toreflux for another 30 min. The mixture was poured on 630 g of a 30%aqueous NaOH-solution until complete hydrolysis, before decanting theorganic phase and washing with water. The crude product was distilled ona Vigreux type column (10 h Pa) to obtain 120.7 g (83.4%) ofhexa-2,4-dien-1-ol.

EXAMPLE 65

[0085] Reduction of jojoba oil

[0086] A 250 ml three-necked flask equipped with a reflux condenser,inner thermometer, syringe pump and magnetical stirrer, was charged with50 g of jojoba oil, 0.2 g of Zn(2-ethyl-hexanoate)₂ (corresponding toabout 4 mole % per ester function), 0.06 g of dimethyl-aminoethanol(about 4 mole %) and 50 ml of toluene. The mixture was heated to refluxand 6.5 g (0.1 mole, about 2.2 equivalents) of PMHS were added over 45min. Reflux was continued for another 30 min, and the reaction mixturewas poured into 50 g of a 30% aqueous NaOH solution. After completehydrolysis, the organic phase was decanted and washed with water. Thethus obtained crude product was distilled in a bulb-to-bulb apparatus at250°/1 h Pa, to obtain 48.4 g (95%) of a product containing 6.4% of(Z)-9-octadecen-1-ol, 59.3% of (Z)-9-icosen-1-ol, 26.8% of(Z)-9-docosen-1-ol and 3.9% of (Z)-9-tetracosen-1-ol.

What is claimed is:
 1. A reductive system capable of being mixedtogether to effect a reduction a carbonyl substrate to a correspondingalcohol, comprising: a silane agent; and an active zinc compound,wherein the zinc compound is monomeric and not a hydride, wherein thesilane and the zinc compound, upon being mixed together, enable thereduction of the carbonyl compounds.
 2. The reductive system of claim 1, wherein the zinc compound is the reaction product formed by thereaction of an oligomeric or polymeric precursor compound of zinc, or adialkylzinc or a diarylzinc compound and a complexing agent.
 3. Thereductive system of claim 1 , wherein the silane agent ispolymethylhydrosiloxane.
 4. The reductive system of claim 1 , whereinthe zinc compound has the general formula ZnX₂L_(n) wherein X is ananion selected from the group consisting of carboxylates; β-diketonates;enolates; amides; silylamides; alkyl, cycloalkyl, alkoxy, aryl, aryloxy,alkoxyalkyl, alkoxyaryl, aralkoxy, aralcoyl and alkylaryl groups havingfrom 1 to 20 carbon atoms; halides; carbonates; and cyanides; and L is aligand selected from the group consisting of amines, polyamines, imines,polyimines, aminoalcohols, amine oxydes, phosphoramides and amides,wherein the anion X and the ligand L can be identical or different, andwherein the ligand L to Zn are present in a ration ranging from 1 to 6.5. The reductive system of claim 4 , wherein the anion X is selectedfrom the group consisting of acetate, propionate, butyrate, isobutyrate,isovalerianate, diethylacetate, benzoate, 2-ethylhexanoate, stearate,methoxyde, ethoxyde, isopropoxyde, tert-butoxyde, tert-pentoxyde,8-hydroxyquinolinate, naphthenate, substituted and unsubstitutedacetylacetonate, tropolonate, a methyl group, an ethyl group, a propylgroup, a butyl group, and an aryl group.
 6. The reductive system ofclaim 4 , wherein the ligand L is selected from the group consisting ofethylenediamine, N,N′-dimethylethylenediamine,tetramethyl-ethylenediamine, ethanolamine, diethanolamine,dimethylaminoethanol, dimethylformamide, dimethylacetamide,hexamethylphosphortriamide, dimethylsulfoxyde or 4-tert-butylpyridine.7. The reductive system of claim 1 , wherein the zinc compound, ispresent in an amount, as expressed in mole percent with respect to thecarbonyl substrate, ranging from 0.1 to 10 percent.
 8. The reductivesystem of claim 1 , wherein the silane agent is used in an essentiallystoichoimetric amount with respect to the carbonyl substrate.
 9. Thereductive system of claim 1 , wherein a molar ratio of the zinc compoundto the silane agent ranges from about 1 to
 2. 10. A reductive system forthe reduction of carbonyl compounds consisting essentially of: aneffective amount of a silane agent to bring about a reduction of acarbonyl substrate to a corresponding alcohol; and an effective amountof a catalyst, wherein the catalyst comprises an active zinc compoundfor catalyzing the reduction, wherein the zinc compound is monomeric andnot a hydride.
 11. The reductive system of claim 10 , wherein the zinccompound is a reaction product formed by the reaction of an oligo- orpolymeric precursor compound of zinc, or a dialkylzinc or a diarylzinccompound and a complexing agent.
 12. The reductive system of claim 11 ,wherein the silane agent is polymethylhydrosiloxane.
 13. The reductivesystem of claim 11 , wherein the zinc compound is a compound of generalformula ZnX₂L_(n), wherein X is an anion selected from the groupconsisting of carboxylates; β-diketonates; enolates; amides;silylamides; alkyl, cycloalkyl, alkoxy, aryl, aryloxy, alkoxyalkyl,alkoxyaryl, aralkoxy, aralcoyl and alkylaryl groups having from 1 to 20carbon atoms; halides; carbonates; and cyanides; L is a ligand selectedfrom the group consisting of amines, polyamines, imines, polyimines,aminoalcohols, amine oxides, phosphoramides and amides, wherein theanion X and the ligand L can be identical or different, and wherein theligand L to Zn are present in a ratio ranging from 1 to
 6. 14. Thereductive system of claim 13 , wherein the anion X is chosen from thegroup consisting of acetate, propionate, butyrate, isobutyrate,isovalerianate, diethylacetate, benzoate, 2-ethylhexanoate, stearate,methoxyde, ethoxyde, isopropoxyde, tert-butoxyde, tert-pentoxyde,8-hydroxyquinolinate, naphthenate, substituted and unsubstitutedacetylacetonate, tropolonate, a methyl group, an ethyl group, a propylgroup, a butyl group, and an aryl group.
 15. The reductive system ofclaim 14 , wherein the ligand L is chosen from the group consisting ofethylenediamine, N,N′-dimethylethylenediamine,tetramethylethylenediamine, ethanolamine, diethanolamine,dimethylaminoethanol, dimethylformamide, dimethylacetamide,hexamethylphosphortriamide, dimethylsulfoxyde or 4-tert-butylpyridine.16. The reductive system of claim 11 , wherein the zinc compound, ispresent in an amount, as expressed in mole percent with respect to thecarbonyl substrate, ranging from 0.1 to 10 percent.
 17. The reductivesystem of claim 11 , wherein the silane agent is used in an essentiallystoichoimetric amount with respect to the carbonyl substrate.
 18. Thereductive system of claim 11 , wherein a molar ratio of the zinccompound to the silane agent ranges from about 1 to
 2. 19. A catalystsystem capable of being reacted together with a silane agent to effect areduction of a carbonyl substrate to a corresponding alcohol, consistingessentially of an effective amount of a catalyst, wherein the catalystis an active zinc compound for catalyzing the reduction, and wherein thezinc compound is monomeric and not a hydride, in combination with acarbonyl substrate to be reduced.
 20. The catalyst system of claim 19 ,wherein the zinc compound is a reaction product formed by the reactionof an oligo- or polymeric precursor compound of zinc, or a dialkylzincor a diarylzinc compound; and a complexing agent.
 21. A reaction productproduced by a catalyzed reduction of a carbonyl substrate by a silaneagent to an alcohol before recovery of the alcohol, consistingessentially of: a catalyst, wherein the catalyst is an active zinccompound, and wherein the zinc compound is monomeric and not a hydride;and an intermediate reaction product comprising the carbonyl substrateand the silane agent.
 22. The reaction product of claim 21 , wherein thezinc compound consists essentially of: an oligo- or polymeric precursorcompound of zinc, or a dialkylzinc or a diarylzinc compound; and acomplexing agent.
 23. The reaction product of claim 22 , wherein thesilane agent is polymethylhydrosiloxane.
 24. A catalyst system capableof effectuating a reduction of a carbonyl substrate by a silane agent,the catalyst system consisting essentially of: an oligo- or polymericprecursor compound of zinc, or a dialkylzinc or a diarylzinc compound;and a complexing agent.
 25. A catalyst consisting essentially of a zinccompound, wherein the zinc compound is monomeric, not a hydride, and isthe reaction product of an oligo- or polymeric precursor compound ofzinc, or a dialkylzinc or diarylzinc compound, and a complexing agent.26. The catalyst of claim 25 , wherein the precursor compound is acompound of the general formula ZnX₂, wherein X is an anion selectedfrom the group consisting of carboxylates, β-diketonates, enolates,amides, silylamides, alkyl, cycloalkyl, alkoxy, aryl, aryloxy,alkoxyalkyl, alkoxyaryl, aralkoxy, aralcoyl and alkylaryl groups havingfrom 1 to 20 carbon atoms, halides, carbonates, and cyanides.
 27. Thecatalyst of claim 26 , wherein the anion X is selected from the groupconsisting of acetate, propionate, butyrate, isobutyrate,isovalerianate, diethylacetate, benzoate, 2-ethylhexanoate, stearate,methoxyde, ethoxyde, isopropoxyde, tert-butoxyde, tert-pentoxyde,8-hydroxyquinolinate, naphthenate, substituted and unsubstitutedacetylacetonate, tropolonate, a methyl group, an ethyl group, a propylgroup, a butyl group, and an aryl group.
 28. The catalyst of claim 26 ,wherein the complexing agent is selected from the group consisting ofamines, polyamines, imines, polyimines, aminoalcohols, amine oxydes,phosphoramides, and amides.
 29. The catalyst of claim 26 , wherein thecomplexing agent is selected from the group consisting ofethylenediamine, N,N′-dimethylethylenediamine,tetramethyl-ethylenediamine, ethanolamine, diethanolamine,dimethylaminoethanol, dimethylformamide, dimethylacetamide,hexamethyl-phosphortriamide, dimethylsulfoxyde or 4-tert-butylpyridine.30. The catalyst of claim 26 , wherein the catalyst is of a generalformula ZnX₂L_(n), wherein X is an anion selected from the groupconsisting of carboxylates, β-diketonates, enolates, amides,silylamides, alkyl, cycloalkyl, alkoxy, aryl, aryloxy, alkoxyalkyl,alkoxyaryl, aralkoxy, aralcoyl and alkylaryl groups having from 1 to 20carbon atoms, halides, carbonates, and cyanides, L is a complexing agentselected from the group consisting of amines, polyamines, imines,polyimines, aminoalcohols, amine oxydes, phosphoramides, and amides,wherein the anion X and the ligand L can be identical or different, andwherein the ligand L and Zn are present in a ratio ranging from 1 to 6.31. A monomeric carboxylate of zinc, with the proviso that a complex[Zn(O₂CCH₃)₂(pyridine)₂] is excluded.
 32. The carboxylate of claim 33 ,the carboxylate comprising at least one of[Zn(benzoate)₂(dimethylaminoethanol)₂],[Zn(benzoate)₂(tetramethylethylenediamine)],[Zn(diethylacetate)₂(1,2-diaminopropane)₂], and[Zn(diethylacetate)₂(2,2′-bipyridyl)].
 33. A process for preparing amonomeric carboxylate of zinc according to claim 32 which comprisesreacting an appropriate oligomeric or polymeric precursor compound ofzinc with a complexing agent under condisitons sufficient to form thecarboxylate.